1. Field of the Invention
The present invention relates to a current limiter, provided in an AC electric path, which operates automatically when an overcurrent (short-circuit current) has started to flow in the electric path, thereby limiting the current value to a safe level instantaneously.
2. Description of the Related Art
Conventionally, in order to protect a power transmission/distribution system, a circuit protection system is employed. In this system, an overcurrent sensor senses an overcurrent in an electric path. A limiting element (LE) is activated by an output from the sensor, thereby separating the electric path from a power source.
In the above circuit protection system, however, in case of a "short-circuit accident", for example, a large current flows through a part of the electric path during a short time period needed for the start of operation of the limiting element. Thus, an electric circuit structure capable of withstanding the large current must be employed. In addition, a limiting element having a large breaking capacity must be used.
For the above reason, it has recently been thought that a current limiter is provided on the power source side of the electric path. The moment an overcurrent starts to flow in the electric path, the current limiter reduces the overcurrent to a safe value. This current limiter must meet the following conditions:
(1) A voltage drop in a normal condition is very small.
(2) An impedance rises quickly the moment a current above a predetermined value starts to flow.
(3) Maintenance is easy and repeated use is allowed.
As a current limiter which meets the above conditions, there is conventionally known a current limiter having a current limiting element formed of a low melting point metal. Specifically, in the current limiter, the limiting element is formed of NaK or a low melting point metal. The limiting element is connected in series to the electric path. The moment a current above a predetermined value starts to flow in the limiting element, NaK is vaporized to obtain a high impedance.
However, in the conventional current limiter having the above structure, a variation occurs in current value at which current limit begins, i.e. vaporization occurs, due to "sealing condition" or "solidification condition" of NaK. Thus, the stability in operation of the current limiter is not reliable.
In the case of alternating current, a current limiter must be provided for each of three phases, resulting in an increase in size of the current limiter. Further, the increase in size raises the running cost of the current limiter.
For example, in the case of a current limiter utilizing superconductivity, if "short-circuit accident" of one phase occurs, the current limiters for the other phases are induced by the strong magnetic flux produced by the superconducting coil for the "accident" phase, and "normal conduction transition" occurs in these current limiters.
Thus, the current limiter for electric power using the conventional system has a low reproductivity of the limit start current value, and low reliability and safety in operation.